1. Field of the Invention
This invention relates to the field of electrical test apparatus and more particularly to a testing apparatus operable to check for short and open circuit conditions in a multiconductor cabling system, installed permanently at the cabling site and capable of testing an entire cabling network as well as a selected cable termination.
2. Prior Art
Computer and telecommunications networks having a central main unit and a plurality of remote units or terminals coupled to the main unit are known in many variations. An example in the computer field is a local area network having one or more central file servers coupled to a plurality of user terminals in a business or office. Another example is a PBX telephone system in which a central switching apparatus is coupled to a plurality of distributed telephone sets.
The main unit is typically located in a closet or computer room, out of the way of the office staff. Wherever it may be located, the main unit is cabled to a number of remote ports, located for example at each desk or office and at other locations where access to the network might be beneficial. In many installations a series of multiconductor cables, each being terminated at a plug or jack, extend through walls, cable ducts and the like from the central unit to the area of the user. Often the plugs are mounted in wall outlets, for example using RJ11 or RJ45 receptacles. A "drop" cable is then used to make the connection between the wall outlet and the computer or other device which is to communicate over the network. For convenience and to accommodate future needs, a large number of wall outlets and cables may be installed in a business, such that a convenient outlet is likely to be available for use, nearby a desk or the like.
In the example of a computer local area network, remote terminals having a video display and limited processing and storage functions can be coupled to the ports as need warrants. The users then can communicate, share devices and otherwise enjoy the additional processing power and storage of the server.
A fault may occur in a device coupled to the network or in the cabling which runs between the central and remote devices. It is easy to misdiagnose a device problem as a connection problem and vice-versa, because often the result is the same: failure to receive accurately at a remote device the information ostensibly transmitted from another device. For example, it is difficult to determine whether a fault in transmission resides in the sender, the receiver, the interface circuits or the terminating plugs/receptacles and cabling between them. Faulty cabling which disrupts network performance is particularly troublesome since the location of the cabling error such as a short or open circuit may occur anywhere, but the symptoms could be observable everywhere.
It is practically a necessity that all network cabling be checked or "rung out" prior to going on-line with the system. Additionally, service technicians generally carry continuity testing apparatus that can be used to identify short and open circuit conditions. Testers are also available to measure resistance, capacitance, inductance, voltage and current. Typically however, cabling testers require two people (one person at each end of a cable) and rely on means for the two to communicate. For single person use, the testers normally require the technician to move back and forth between points at which the cable conductors can be accessed. For example, a simple portable continuity tester may require the user to detach the cable at both ends, check for shorts by measuring continuity between all the conductors in pairs, then short conductors at one end of a cable and measure continuity between the conductors at the opposite end.
Separate components adapted to facilitate this operation can be coupled to the opposite ends of a cable, using the same form of connector as the device which normally uses the port. One component is a shorting plug or diode arrangement; the other component generates a signal on certain of the conductors and checks for the signal to appear on others of the conductors. Provided the cable connections are correct along the cable path, which may include intermediate connections, and are not shorted, signals generated and sent through the cable and returned and read on the conductors where they are expected to appear only. The diode arrangement allows for signals at different polarities to identify shorts and opens on particular conductors to which current is routed or not routed as a function of the voltage bias across the respective diodes.
Another form of known tester is a listening device which is coupled serially with the cable and which senses data transmitted along the cable, either from a terminal or from a test apparatus plugged into a wall port in place of the terminal.
Other cable testing devices are disclosed in a number of U.S. patents, for example Simmonds U.S. Pat. No. 3,944,914. Simmonds discloses apparatus for detecting faults in multiconductor cable having paired conductors. The apparatus described in Simmonds is portable and can be coupled temporarily to the terminus of a port. A switch in the device selects which pair of conductors in the multiconductor cable will be tested. The device charges the conductors and measures capacitance, and the measured capacitance is compared to known or expected values. Measured values departing from nominal values indicate a fault in the cable.
Donahue U.S. Pat. No. 4,837,488 discloses another portable cable tester. Donahue uses a first unit including a power supply for mating with an end of a cable and a second unit for mating with an opposite end of the cable. Jumpers provide paths for current to ensure that the test signal is routed through all four (4) wires of a network cable.
Fore U.S. Pat. No. 4,943,993 discloses a cable pair tester that initially relies on a lack of electrical continuity between cable pairs. Upon command from the cable tester, electrical continuity is established between selected ones of the cable pairs and a test signal is applied to verify the continuity of the conductors.
Haferstat U.S. Pat. No. 5,027,074 discloses a portable multiconductor cable tester including a transmitter for connection to one end of a cable and a receiver for connection to an opposite end of the cable. The transmitter sends data in the form of voltage pulses, which are intended for receipt by the receiver, where the data is checked against expected values. The receiver monitors the pulses and feeds the data to an on-board microprocessor for processing, and display of the results on an LCD display.
It would be advantageous to provide a cable testing device which is automated but does not require special steps for coupling portable devices to each end of the cables to be tested. According to the present invention, such a cable tester is achieved in that means are permanently installed in the wall outlets or the like, which can be used later as cable terminations when the cable is to be tested. Such an arrangement would appear to have two major drawbacks, namely the cost of the permanently installed termination circuitry, and the fact that termination circuits would presumably interfere with the basic use of the cable for data transmission, or at least would detract from the performance characteristics of the port. However, according to the invention a mechanically operated electrical interlock disengages the termination circuits from the cable when a connector is plugged in, and engages the termination circuits otherwise. The termination circuits are inexpensive enough for permanent installation and do not interfere with use of the cable. A central testing means enables an investigation of the termination circuits in an automated manner, providing an on-site apparatus for testing of an entire cabling network or a single addressed cable from a central location.